This invention relates to a temperature actuated shutdown assembly which is useful for providing highly reliable and therefore inherently safe automatic actuation of liquid metal nuclear reactor core safety devices upon attainment of a preset temperature. More specifically, this invention provides automatic insertion of neutron absorbers into the core of a liquid metal nuclear reactor which is in danger of overheating due to excessive power for a given coolant flow.
Nuclear reactor cores are usually protected against excessive power by raising or lowering the number of neutrons available to cause fissions, the number of fissions per unit time being a direct measure of the energy released as heat in the core. The number of neutrons can be sharply reduced by insertion of a neutron absorber such as boron carbide. For rapid insertion into the core such absorbers usually rely on mechanical or magnetic latches which require signals, such as the cessation of electrical power, to release them. The assembly described herein is simple and therefore reliable, and purely mechanical in nature. Its mass is small and thus the assembly will respond rapidly to changes in temperature.
Changes in temperature of the assembly may be brought about by (a) an increase in the rate with respect to time at which energy flows into the assembly from another, hotter body, or (b) a decrease in the rate with respect to time at which energy flows out of the assembly to another, cooler body, or (c) an increase or decrease in the rate with respect to time of direct generation of energy as heat in the assembly by increasing or decreasing the number of fissions occurring in the assembly. In the first two modes, the driving force is the temperature difference between the assembly and its surroundings, while in the third mode, both the temperature difference and the rate at which fissions are taking place control the heating or cooling of the assembly. Any increase in the heat addition or generation rate without a corresponding increase in the heat removal rate leads to heating of the assembly and, if severe enough to constitute a danger or unsafe condition, eventual actuation of the assembly. Similarly, a decrease in the heat removal rate without a decrease in the heat addition or generation rate conduces to the same result. In a nuclear reactor, an increase in the rate of heat addition will be brought about by an increase in reactor power, i.e., the number of fissions per unit time. A decrease in the rate of heat removal will be brought about by a decrease in or complete loss of coolant flow. Increased heat generation in the assembly itself requires the presence of fissile material in the assembly which will respond to a higher fission rate in the rest of the reactor.
Relying upon heat generation within the assembly itself is especially useful because where coolant flow remains constant an increase in reactor power is a necessary prelude to overheating; therefore the assembly will actuate in response to the earliest indication of potential danger, since the increased fission rate in the assembly will cause the temperature of the assembly to rise due to heat generation as well as an increase in the heat addition rate due to the derivative effect of rising temperature in core components or coolant.
In the case of loss of or reduction in coolant flow, the assembly must depend on the decrease in heat removal rate. However, its small mass and intimate contact with coolant and core structure will still result in a rapid temperature response.